A Crest factor or peak-to-average-power-ratio (PAPR) describes for a signal constellation set (SCS) or constellation scheme a ratio between a maximum power a single constellation point in the signal constellation set can have to an average power of all constellation points of the SCS. A high Crest factor may result in problems for amplifiers, in particular in terms of linearity.
Data transmissions between radio devices constantly have to be improved. In particular, it may be desirable to provide a Crest factor resulting in an improved data transmission.
In the following detailed description, reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.
In the following, various modulation schemes, methods for providing modulation schemes and devices for modulating data according to such modulation schemes are described separately or with reference to each other. It is understood that comments made in connection with a described method may also hold true for a corresponding device configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such a unit is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically noted otherwise.
Modulation may be seen as the process of conveying a message signal, for example a digital bit stream, inside another signal that may be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example a low-frequency audio signal into a radio-frequency (RF) signal. In radio communications, cable TV systems or the public switched telephone network (PSTN), electrical signals may be transferred using a limited passband frequency spectrum with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband.
An aim of digital modulation may be to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (in which a bandpass filter limits the frequency range between 300 Hz and 3400 Hz) or over a limited radio frequency band. In digital modulation, an analog carrier signal may be modulated by a discrete signal. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet).
Digital modulation schemes or techniques may be based on keying. In the case of phase-shift keying (PSK), a finite number of phases are used. In the case of frequency-shift keying (FSK), a finite number of frequencies are used while in amplitude-shift keying (ASK), a finite number of amplitudes are used. Further, in the case of quadrature amplitude modulation (QAM), a finite number of at least two phases and at least two amplitudes are used while in QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes and summed. This can be seen as a two-channel system, each channel using ASK wherein the resulting signal is equivalent to a combination of PSK and ASK. Each of these phases and amplitudes may be assigned a unique pattern of binary bits. Usually, each phase and amplitude encodes an equal number of bits. This number of bits includes a “symbol” represented by the particular phase and amplitude. If the alphabet consists of M=2N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is fS symbols/second (or baud), the data rate is NfS bit/second. For example, with an alphabet consisting of 16 alternative symbols, each symbol represents four bits. Hence, the data rate is four times the baud rate. In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet may be represented on a constellation diagram (i.e. as a constellation scheme), showing the amplitude of the I signal at the x-axis and the amplitude of the Q signal at the y-axis for each symbol.
In the following, constellation diagrams are described. A constellation diagram or a constellation scheme may correspond to a representation of a signal modulated by a digital modulation scheme such as QAM or PSK. It may display the signal as a two-dimensional scatter diagram (i.e. a mathematical diagram using Cartesian coordinates to display values for two variables for a set of data) in the complex plane at symbol sampling instants. The constellation diagram may thus represent the possible symbols that may be selected by a given modulation scheme as points in the complex plane. Measured constellation diagrams may be used to recognize the type of interference and distortion in a signal. Since the symbols are represented as complex numbers, they may be visualized as points in the complex plane. The real and imaginary axes may be called the in phase or I-axis and the quadrature or Q-axis, respectively. Plotting several symbols in a scatter diagram results in the constellation diagram. The points on a constellation diagram may be referred to as constellation points. They are a set of modulation symbols including the modulation alphabet. Also a diagram of ideal positions, a signal space diagram, in a modulation scheme may be called a constellation diagram. In this sense, the constellation does not correspond to a scatter diagram, but a representation of the scheme itself.
For example, modulation schemes may be QAM, a combination of PSK and ASK, e.g. 16QAM, 64QAM, 1024QAM or 4096QAM. Modulation schemes like 16QAM and 64QAM may be considered as higher order signal constellation sets for wireless communications, e.g. high speed data packet access (HSDPA) and long term evolution (LTE) while modulation schemes like 1024QAM and 4096QAM may be considered as higher order signal constellation sets for wireline communication, e.g. digital subscriber line (DSL). Even modulation schemes having more constellation points may be used due to lower noise. For limiting bit and block errors, modulation schemes may be applied using a Gray mapping. In Gray mapped modulation schemes the bit patterns of neighboring points usually differ by only a single bit.
In the following, various modulators, demodulators and devices for modulating data are described. A modulator or a device for modulating data may be implemented in a transmitter or transceiver. In order to transmit data, the modulator or the device for modulating data may perform one or more of the following steps:                1. Group incoming data bits into codewords or bit patterns, one for each symbol that will be transmitted.        2. Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values according to a modulation scheme.        3. Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum of the equivalent low pass signal, typically using digital signal processing.        4. Perform digital-to-analog conversion (DAC) of the I and Q signals, e.g. by using digital signal processing (DSP).        5. Generate a high-frequency sine wave carrier waveform, and/or a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine wave form with the I and Q signals, resulting in that the equivalent low pass signal is frequency shifted into a modulated passband signal or RF signal. This may be achieved by using DSP technology.        6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum.        
At a receiver side a demodulator or a device for demodulating data may be implemented. A device for modulating data may include a device for demodulating data, in this case it is called a modem (device for modulating/demodulating data). A device for demodulating data may perform one or more of the following steps:                1. Bandpass filtering.        2. Automatic gain control (AGC) to compensate for attenuation, for example fading.        3. Frequency shifting of an RF signal to the equivalent baseband I and Q signals or to an intermediate frequency (IF) signal by multiplying the RF signal with a local oscillator sinewave and cosine wave frequency.        4. Sampling and analog-to-digital conversion (ADC), for example by means of undersampling.        5. Equalization filtering, for example using a matched filter, compensation for multipath propagation, time spreading, phase distortion and frequency selective fading to avoid intersymbol interference and symbol distortion.        6. Detection of the amplitudes of the I and Q signals or the frequency or phase of the IF signal.        7. Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.        8. Mapping of the quantized amplitudes, frequencies or phases to codewords (bit patterns or bit groups) according to a modulation scheme.        9. Parallel-to-serial conversion of the codewords into a bit stream.        10. Passing the resultant bit stream on for further processing such as removal of any error-correcting codes.        
In the following, various channel codes, convolutional codes, Digital Video Broadcasting Satellite 2nd generation (DVBS2) codes, Turbo codes and low density parity check (LDPC) codes are described.
In digital communications, the term “channel code” usually refers to a forward error correction code and bit interleaving in communication and storage, wherein the communication media or storage media may be viewed as a channel. The channel code may be used to protect data sent over it for storage or retrieval even in the presence of noise (errors). Channel codes may be made up of two main type of codes, convolutional codes and block codes.
Convolutional codes are usually used for real-time error correction and may convert a data stream into one single codeword. A Viterbi algorithm provides a basis for the main decoding strategy of convolutional codes. The encoded bits depend not only on the current informational k input bits, but also on past input bits, i.e. the memory of the code. A convolutional code is a type of error-correcting code in which each m-bit information symbol (each m-bit string) to be encoded may be transformed into an n-bit symbol, where m/n corresponds to the code rate (n=m) and the transformation is a function of the last k information symbols wherein k is the constraint length or the memory parameter of the code.
Block codes may be based on a finite field arithmetic and abstract algebra. Block codes accept a block of k information bits and return a block of n coded bits. Block codes are usually used for correcting or detecting errors in a data transmission. Commonly used block codes are Reed-Solomon codes, BCH codes, Golay codes and Hamming codes.
DVB-S2 codes are related to the Digital Video Broadcasting Satellite-Second Generation (DVB-S2) which is a digital television broadcast standard that has been designed as a successor for the DVB-S (Digital Video Broadcasting Satellite) system. Compared to the DVB-S standard, DVB-S2 provides a coding scheme based on a modern LDPC code as well as Variable Coding and Modulation (VCM) and Adaptive Coding and Modulation (AOM) which allow optimizing bandwidth utilization by dynamically changing transmission parameters.
DVB-S2 provides VCM to optimize bandwidth utilization based on the priority of the input data and ACM to allow flexibly adapting transmission parameters to reception conditions of terminals, e.g. switching to a lower code rate during fading. There are four modulation modes: QPSK, 8PSK, 16APSK and 32APSK. DVB-S2 may provide improved rolloff factors of 0.20 and 0.25 in addition to the roll-off factor of DVB-S which is 0.35. DVB-S2 provides improved coding by concatenating a large LDPC code with an outer BCH code to achieve quasi-error-free (QEF) reception conditions on an additional white gaussian noise (AWGN) channel. The outer code is introduced to avoid error floors at low bit-error rates. A single forward error correction (FEC) frame may have either 64800 bits (normal) or 16200 bits (short). DVB-S2 provides several code rates for a flexible configuration of transmission parameters, which are r=1/4, 1/3, 2/5, 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 8/9, and 9/10. Code rates r=1/4, 1/3 and 2/5 have been introduced for poor reception conditions in combination with QPSK modulation. An optional input stream synchronization may provide a constant end-to-end delay.
Turbo codes may refer to a class of high-performance FEC codes and were designed for achieving reliable information transfer over bandwidth- or latency-constrained communication links in the presence of data-corrupting noise. There are many different instantiations of turbo codes, using different component encoders, input/output ratios, interleavers and puncturing patterns.
An LDPC code may correspond to a linear error correcting code that is constructed using a sparse bipartite graph. LDPC codes are capacity-approaching codes, which means that practical constructions exist that allow the noise threshold to be set very close to the theoretical maximum (the Shannon limit) for a symmetric memory-less channel. The noise threshold defines an upper bound for the channel noise, up to which the probability of lost information may be made as small as desired. Using iterative belief propagation techniques, LDPC codes may be decoded in time linear to their block length. For large block sizes, LDPC codes may be constructed by first studying the behavior of decoders. As the block size tends to infinity, LDPC decoders may be shown to have a noise threshold below which decoding is reliably achieved and above which decoding is not achieved. This threshold may be optimized by finding the best proportion of arcs from check nodes and arcs from variable nodes.